This article by Judi Rawlings, was published in the March, 2020 edition of Bioenergy Insight magazine, under the title of Particle Size Matters. Bioenergy Insight is a publication of Woodcote Media Ltd in Surrey, UK.
Markets for wood waste include feedstock for engineered woods, landscape mulch, soil conditioner, animal bedding, compost additive, sewage sludge, and boiler fuel. All these end uses have similar processing requirements in that the wood waste has to be separated from other wastes, cleaned by removing contaminants and fasteners, and, in some cases, processed through grinding or chipping. The final use of the wood waste often determines how clean and consistent the feedstock must be (California Integrated Waste Management 2001).
During our 40 years designing wood waste grinding systems we have found that most of our clients are looking to get the smallest end particle size on a single pass “one stage pass through the grinder”. We often suggest that this is not always the most economical option in the long run. Running the grinder and related equipment at full surge capacity results in higher consumption of spare parts and maintenance costs. And depending if you’re using electricity or diesel fuel these costs add up over time.
During the preliminary design there are many factors that should be considered. Such factors include species of wood, moisture content, diameter and length of incoming wood waste, metal contamination and most importantly the required end product size.
For high capacity wood handling operations that typically receive a large variety of sized material such as railroad ties, pallets, construction, demolition and hurricane debris, Rawlings recommends wood waste systems that incorporate a combination of both stationary horizontal feed and vertical feed grinders. Horizontal grinders are often utilized as the primary while the vertical grinders are used to produce a smaller more consistent particle size.
If the wood handling system is designed to receive a steady stream of bark or sawmill waste a vertical feed hog is the preferred option. Typically in this scenario most of the incoming material is preprocessed further upstream and it already sized to be processed through this type of grinder. Rawlings offers both vertical and horizontal models in various sizes to meet these requirements. Each system can be designed with a combination of metal protection, product screening, separation, conveyors, and storage handling customized to each specific operation.
Increasing the use of Alternative fuels at cement plants.
During this project design it was determined that the type of biomass fuel that would be readily available would be biomass residues that are normally disposed in a landfill .The project implementation would dispose these wastes in a sustainable manner and recover their energy content. The aim of the project is to substitute as much coal as possible with alternative fuels. This will result in significant long-term reductions of C02 and allow the company to collect and use green energy credits to offset the C02 emissions.
Traditionally, solid fuels such as coal, pet coke and lignite are used in the burning process in clinker kilns. However, kilns are particularly well-suited for alternative fossil fuels and biomass. In North America, 60 per cent of clinker production uses alternative fossil fuel, for up to a maximum of 40 per cent of energy, but very little biomass until recently.
Rawlings assisted with the design of a custom wood handling system capable of receiving, processing and storing up to 350 Tons of processed wood waste to be burned along with coal to reduce the amount of greenhouse emissions. Once the wood waste has been processed through the Rawlings horizontal wood hog, the metal is removed by an overhead self -cleaning magnet and then conveyed to a 250 Ton walking floor stoker storage system. To ensure the optimal size end product the wood waste is then processed over two vibrating finger screens and then transferred to the kilns via a blower system.
Another use for wood residuals is utilized in making Compost.
This Compost facility located in Canada takes food and wood waste from the community, in addition to bio solids from the sewage treatment plant, and processes them into profitable, nutrient-rich Class A compost or biofuel. The system relies on fully enclosed flow through tunnels to transform organic wastes into a fine soil-like material over a 14-day period. By controlling oxygen, moisture and temperature, the in-vessel system provides control over the decomposition of organic matter that exceeds all standards set for composting facilities in North America, the United Kingdom and Europe. Compact, fully enclosed and modular in design, the system is not affected by the weather.
Utilizing a Rawlings 460 Electric Stationary Wood Grinder and the state of the art in-vessel composting tunnels, assist the community towards its goal of zero waste. Rawlings assisted with the design of the wood waste processing system to grind the incoming wood waste. This system is complete with a belt in-feed and out-feed conveyor, metal detector, work platform, Rawlings Horizontal grinder, overhead self-cleaning magnet and electrical control panel to operate the system.